Did the West Antarctic Ice Sheet Collapse during MIS-5e? A Paleotemperature Analysis Using the Morphometrics of Southern Ocean Diatom Fragilariopsis kerguelensis.

Here, we attempt to quantify ocean temperatures in the Amundsen Sea in MIS-5 (~71-130ka) to determine if MIS-5e (115-130ka) was warm enough to sustain a complete collapse of the West Antarctic Ice Sheet (WAIS). MIS-5e saw a global rise in sea level, yet it is unclear if the WAIS completely collapsed during this period. We define a complete collapse of the WAIS as the connection of the Amundsen, Ross, and Weddell Seas per Kinsglake et al (2018). We collect these temperatures using the bimodal morphometrics of the Southern Ocean diatom Fragilariopsis kerguelensis, which displays a high rectangularity mode at lower temperatures. This high rectangularity morphotype is shown to be present in much lower concentrations in warmer waters, thus high concentrations of the low rectangularity mode are used as a proxy for ocean warming during interglacial periods. An increase in ocean temperature of ~2°C would likely be enough to trigger a runaway retreat of the Thwaites and Pine Island Glaciers, which buttress and stabilize the rest of the WAIS (Turney et al, 2020; Mas e Braga, 2021). In this study, we analyze F. kerguelensis valves collected from drill core PC496 on 2008 James Clark Ross (JCR) expedition 179 to the Amundsen Sea to determine summer sea surface temperatures (SSSTs) present throughout MIS-5e. While we find an increase in both absolute diatom abundance and paleotemperature during MIS-5e, it is estimated that SSSTs reach a maximum of 0.78°C in the Amundsen Sea. CTD data collected in 2001 close to PC496 recorded surface temperatures between -0.2°C and -0.5°C, meaning ocean temperatures would not have reached the +2°C-from-modern requirement for total WAIS collapse during MIS-5e. Our findings align with similar work completed on drill core U1533 collected on IODP expedition 379 to the Amundsen Sea, in which temperatures were found to have reached a maximum of 0.80°C during MIS-5e (Mastro et al, 2021 [in prep.]). Thus, there is insufficient evidence recorded in the biometrics of F. kerguelensis to support a total collapse of the WAIS in MIS-5e. However, while the Ross, Weddell, and Amundsen Seas may not have connected in a complete collapse of the WAIS, each of these areas may have seen extensive ice sheet retreat under these warming conditions, evidence of which may be preserved in the diatom record and awaiting future analysis.